A simple fifth order low pass filter to reduce tones and noises from the FM stereo receiver. The filter frequency of 57kHz may be a bit close to the mpx stereo band which runs from 10Hz to 53kHz. But this should not be an issue due to the compensation circuitry on the stereo demodulator.
A modified version of the filter will be used for output filtering of the MPX demodulator. This is done using a LA3401 chip. This modified filter will be using a 5th order Bessel filter with -1dB point at 15kHz.
FM radio broadcasting modulates the stereo information around 38kHz with a double sideband. This stereo information is demodulated using the 19kHz pilot tone that is also transmitted in FM stereo. In order to prevent demodulation errors in the MPX demodulator, the 3rd and higher harmonics of the 38kHz signal (generated from the 19kHz pilot tone) need to be suppressed. Because the stereo information is double sideband modulated, the filter can be placed close to the band. Thus a -3dB of 57kHz, the location of the RDS tone, was chosen as the cut off frequency. Since the pass band needs to have an flat amplitude response, but high attenuation is needed at 114kHz, a butterworth is used.
The implementation of the active filter is done with the sallen key topology, for ease of design. The improved suppression of the multiple feedback is not needed since the signal to be suppressed is relatively low in frequency
After completing, and testing of the design. It struck me, that U2B would have been placed better at the input. If U2B would havebeen placed in front of R1/C3 filter pole, the filter would have been completely isolated from other circuitry.